In "Who, That, Which" I explain which of these pronouns are appropriate for different kinds of antecedents. But another problem people often have when using relative pronouns is deciding when a relative clause is restrictive or nonrestrictive.

Whether a relative clause is restrictive or nonrestrictive matters for two reasons:

1. Restrictive relative clauses are not set off by commas, while nonrestrictive relative clauses are.

2. As a general rule, the pronoun "that" should be used for restrictive relative clauses, and "which" should be used for nonrestrictive relative clauses.*

In "The Loyal Apposition" I offer guidelines for determining whether an appositive is restrictive or nonrestrictive. These guidelines are equally valid for identifying restrictive and nonrestrictive relative clauses.

A. NONRESTRICTIVE RELATIVE CLAUSES

If a relative clause adds parenthetical, nondefining information, it is nonrestrictive. A nonrestrictive (parenthetical) element is set off by commas, as in these examples.Ex: "Mr. Smith, who is a well-respected lawyer, has just retired from active practice." "Professor James, who is an expert in Victorian poetry, will be giving a lecture tonight."

MORE EXAMPLES:Ex: "Your task, which is to seek out new civilizations and boldly go where no man has gone before, will probably occupy the rest of your adult life.""Kofi Annan, who is the current U.N. Secretary General, has spent much of his tenure working to promote peace in the Third World.""This is Jennifer, who is my college roommate's youngest daughter.""That book, which is the novel I was reading last week, is the one I meant for you to take to the beach with you."

B. RESTRICTIVE RELATIVE CLAUSESWhen the relative clause limits or restricts the noun or noun substitute (substantive) it modifies, it is restrictive, and it is not set off by commas.Ex:"My brother that lives in Arizona is named Pat."

In this sentence the clause "that lives in Arizona" is needed to specify which brother, since the reader has no other way of knowing how many brothers the writer has or which brother is being referred to. (One way to think of the issue of restrictive and non-restrictive elements is that a restrictive element provides information that is necessary to narrow the field of candidates down to one.)

But check out this example: "My other brother, who lives in Texas, is named Sam."

In this sentence the first substantive, the noun phrase "My other brother," conveys the information that the writer has only two brothers, and it also specifies which of those two brothers is being referred to, so the fact that he lives in Texas is extra information--not necessary for specifying which of two brothers is being referred to. In fact, although the brother's name is given in this sentence, the name itself isn't actually needed to narrow the field of candidates to one: the phrase "my other brother" indicates that the writer has only two brothers, and it also specifies which of those two brothers he is referring to. (Obviously, the would not say "my other brother" except in a context where he has just referred to the first brother.)

If the relative clause "who lives in Texas" were treated as restrictive, then the sentence would convey the information that the writer has two brothers who live in Texas, and that would only make sense if another brother living in Texas had already been mentioned:Ex: "I have two brothers that live in Texas. One is named Eric. My other brother who lives in Texas is named Sam."

Judgment Calls

Sometimes whether to treat a relative clause as restrictive or nonrestrictive is simply a judgment call.Ex: "My sister, who is even deafer than I am, is named Linda."

In this example, the information that Linda "is even deafer than I am" is extra. Since the main clause names the sister as Linda, the information in the relative clause is not necessary to identify which of the writer's sisters he is referring to.

But this relative clause could be treated as restrictive, giving the sentence a slightly different meaning:Ex: "My sister who is even deafer than I am is named Linda."

This version of the sentence indicates that the purpose is to call the reader's attention to a specific sister--the one "who is even deafer than I am," as opposed to one or more other sisters who are not.

This is an important point: sometimes whether a clause is restrictive or nonrestrictive is determined by subtleties of meaning, and it is up to the writer to make sure that the sentence says exactly what he means.Ex: "My youngest sister, who has three children, is named Carol."

In this example, the phrase "my youngest sister" doesn't tell exactly how many sisters I have (though it does indicate three or more, because "youngest" is in the superlative form). But it does specify exactly which one I am referring to, as there can be only one "youngest sister," so the information about the three children is extra information, not needed to specify which one of however many sisters I am referring.Ex:"My daughter recently attended a Shakespearean play that was being performed at the rebuilt Globe Theater in London."

In this case, the relative clause "that was being performed at the rebuilt Globe Theater in London" is restrictive because it is being used to specify which Shakespearean play she attended. There are many Shakespearean plays, and they are being performed all the time in many places. The relative clause narrows the field of candidates down to one.

But this is another sentence where the relative clause could be treated as nonrestrictive, giving a slightly different meaning to the sentence:Ex: "My daughter recently attended a Shakespearean play, which was being performed at the rebuilt Globe Theater in London."

This version of the sentence emphasizes the fact that the play was being performed in the rebuilt Globe Theater, not which play she attended.

QUICK SUMMARY:

1. If the clause is restrictive, choose "that" over "which" (about a 99% rule), and don't set the clause off with commas. 2. If the clause is nonrestrictive, choose "which" over that" and do set the clause off with commas.**

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* About a 99% rule.** The relative pronoun "who" can govern both restrictive and nonrestrictive relative clauses.http://grammartips.homestead.com/nonrestrictivecommas.html